So there’s little evidence to suggest that we need to avoid protein and fat. But what about the claims Ornish makes about the success of his own diet—do they hold up to scrutiny? Not exactly. His famous 1990 Lifestyle Heart trial involved a total of 48 patients with heart disease. Twenty-eight were assigned to his low-fat, plant-based diet and 20 were given usual cardiac care. After one year those following his diet were more likely to see a regression in their atherosclerosis.

But here’s the thing: The patients who followed his diet also quit smoking, started exercising and attended stress management training. The people in the control group were told to do none of these things. It’s hardly surprising that quitting smoking, exercising, reducing stress and dieting—when done together—improves heart health. But fact that the participants were making all of these lifestyle changes means that we cannot make any inferences about the effect of the diet alone.